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This invention relates to a vibration damping silicone composition, more specifically a vibration damping silicone composition of superior storage stability and vibration damping properties.
Silicone compositions containing silicone oils and fillers are vibration damping materials due to the low viscosity temperature dependency of the silicone oils and their large bulk compressibility. These compositions have been used for vibration damping members of precision equipment such as precision measurement devices, magnetic disks, magneto optical disks, other magnetic signal reading devices, compact disks, laser disks, and other optical signal reading devices. For example, a silicone composition prepared by mixing a silicone resin powder and a calcium carbonate powder with a silicone oil is described in Japanese Patent Application Publication No. 2000-080277. However, there exists a need for vibration damping silicone compositions that would possess superior vibration damping characteristics without using expensive silicone resin powders
Therefore, it is an object of the invention to provide a vibration damping silicone composition having superior storage stability and vibration damping properties.
These and other features of the invention will become apparent from a consideration of the detailed description.
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The invention is directed to vibration damping silicone compositions containing (A) 100 parts by weight of a silicone oil, (B) 20-250 parts by weight of a heavy calcium carbonate powder, (C) 10-100 parts by weight of a light calcium carbonate powder, and (D) 0.01-20 parts by weight of a fatty acid or a fatty acid derivative.
The silicone oil component (A) functions as the medium for dispersing micropowders of component (B) and component (C), and is typically an organopolysiloxane that is a liquid at room temperature. Groups bonded to silicon atoms in the organopolysiloxane are exemplified by monovalent hydrocarbon groups such as methyl, ethyl, and propyl, and other alkyl groups; vinyl, butenyl and other alkenyl groups; phenyl, tolyl and other aryl groups; and 3,3,3-trifluoropropyl and other halogenated alkyl groups. Hydroxyl groups, or methoxy, ethoxy and other alkoxy groups can be substituted for some of the hydrocarbon groups. Alkyl groups are most preferable, and methyl is particularly preferable because of the excellent storage stability of such compositions.
The molecular structure of the organopolysiloxane can be linear, partially branched linear, branched, or cyclic, but a linear structure is preferred. The kinematic viscosity of component (A) at 25xc2x0 C. should be 100-1,000,000 mm2/s, preferably 500-500,000 mm2/s. When the kinematic viscosity at 25xc2x0 C. is less than 100 mm2/s, component (B) and component (C) are difficult to maintain in a uniformly dispersed state in component (A); whereas when it exceeds 1,000,000 mm2/s, the handling properties deteriorate, and component (B) and component (C) become difficult to disperse in component (A). Component (A) is exemplified by dimethylpolysiloxanes end blocked by trimethylsiloxy groups, dimethylpolysiloxanes end blocked by dimethylvinylsiloxy groups, dimethylpolysiloxanes end blocked with silanol groups, and copolymers of methylphenylsiloxane and dimethylsiloxane end blocked by trimethylsiloxy groups.
To increase the long term storage stability and reliability of component (A), it is preferred that it contain less than 1,000 ppm of organosiloxane oligomers with not more than 20 silicon atoms. Silicone oils with reduced amounts of such oligomers can be obtained by subjecting silicone oils with oligomer content of 10,000-40,000 ppm to stripping after ordinary equilibrium polymerization to reduce the content of oligomer. For example, such silicone oils can be produced by processes in which oligomers are removed by (i) a thin film evaporator at 270-350xc2x0 C. and 0.1-15 mmHg, (ii) processes in which oligomers are extracted and removed with alcohols such as methanol, ethanol, propanol, and butanol, or ketones such as acetone and methylethylketone, or (iii) by reprecipitation methods.
Heavy calcium carbonate powder component (B) is the filler used for the vibration damping silicone composition. It is also referred to in the art as crushed calcium carbonate, and is typically produced by grading crushed white lime. This type of heavy calcium carbonate powder is commercially available under names such as WHITONE P-30 and WHITONE P-305, by Toyo Fine Chemicals, Co., Ltd.; and NANOX# 30 by Maruo Calcium Co., Ltd. Heavy calcium carbonate powder with surfaces treated with a fatty acid or a fatty acid derivative are particularly preferred because of the superior stability of dispersions of component (B) in component (A). Some examples of suitable fatty acids which can be used for this surface treatment are caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, cerotic acid, behenic acid, elaidic acid, and arachidic acid. Fatty acid derivatives which can be used are exemplified by alkali metal salts, alkaline earth metal salts, or metal salts of such fatty acids.
The average particle size of the heavy calcium carbonate powder is preferably 0.01-300 xcexcm, and more preferably 0.01-100 xcexcm. The amount of component (B) used in the composition is 20-250 parts by weight, preferably 50-200 parts by weight, per 100 parts by weight of component (A). When there is less than 20 parts by weight of component (B), the vibration damping characteristics of the composition of the invention deteriorates, and when the amount exceeds 250 parts by weight, its operating properties deteriorate.
Light calcium carbonate powder component (C) is another filler which is used with component (B). Light calcium carbonate powder is referred to in the art as precipitated calcium carbonate, and is typically produced by dehydrating and drying a light calcium carbonate slurry obtained by reacting dense limestone with carbon dioxide gas. Again, the surface of the powder component (C) can be treated with any of the fatty acids or fatty acid derivatives noted above.
These types of light calcium carbonate powder are commercially available under names such as HAKUENKA CC from Shiraishi Calcium Co., Ltd., and CALFINE 200 from Maruo Calcium Co., Ltd. The average particle size of component (C) is preferably 0.01 xcexcm-300 xcexcm, more preferably 0.01-100 xcexcm. The amount of component (C) used in the composition is 10-100 parts by weight, preferably, 15-80 parts by weight, per 100 parts by weight of component (A). When less than 10 parts by weight of component (C) is used, the vibration damping properties of the composition of the invention deteriorate, and when the amount exceeds 100 parts by weight, its operating properties deteriorate.
The total amount of the heavy and light calcium carbonate powders components (B) and (C) preferably exceeds 150 parts by weight, more preferably 170 parts by weight, per 100 parts by weight of component (A).
Fatty acid or fatty acid derivative component (D) is present to improve the storage stability and dispersibility of the vibration damping silicone composition of the invention. Component (D) can comprise a fatty acid such as caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, cerotic acid, behenic acid, elaidic acid, and arachidic acid. Component (D) can also comprise a fatty acid derivative such as an alkali metal salt, an alkaline earth metal salt, or a metal salt of such fatty acids. Fatty acids with 6-31 carbon atoms or their derivatives are preferred. One example of a preferred fatty acid derivative is zinc stearate. The amount of component (D) used in the composition is 0.01-20 parts by weight, preferably 0.1-15 parts by weight, per 100 parts by weight of component (A). When less than 0.01 parts by weight of component (D) is used, the storage stability of the composition of the invention deteriorates, and when the amount exceeds 20 parts by weight, its vibration damping characteristics deteriorate.
While silicone compositions according to the invention contain components (A)-(D), inorganic powders other than calcium carbonate powders (B) and (C), can be included as an additional component. Some examples of suitable inorganic powders which can be used include silica micropowder, glass micropowder, clay, bentonite, diatomaceous earth, quartz powder, talc, titanium oxide powder, tin oxide powder, aluminum oxide powder, and iron oxide powder. In addition, there can be included components such as anti-oxidants, rust preventatives, flame retardants, pigments, and dyes, provided the component does not impair the function of the composition of the invention.
Silicone compositions of the invention are produced by uniformly mixing components (A)-(D). This can be accomplished with a ball mill, a vibrating mill, a kneader mixer, a screw extruder, a paddle mixer, a ribbon mixer, a Banbury mixer, a Ross mixer, a Henschel mixer, a flow jet mixer, a Hubbard mixer, or a roll mixer. The composition can be heated during the mixing process, preferably at a temperature of 30-200xc2x0 C.
Silicone compositions according to the invention exhibit superior storage stability and vibration damping properties, and are characterized by low temperature dependence. The silicone compositions can be used in shock absorbers by filling elastic containers such as rubber bags or rubber cylinders with the composition. The silicone compositions are particularly suitable for shock absorbers used in compact disk players, compact disk changers, mini-disk players, and car navigation devices.